When the heart or other muscle contracts, the body generates a very low amplitude electric signal known as a biopotential signal. It is well known that this biopotential signal can be electronically detected on the surface of a person's skin. Because the heart expands and contracts in a regular rhythm, it generates a periodic biopotential signal. Thus, certain periodic voltage fluctuations on a person's skin correspond to heartbeat.
Devices for monitoring the heart rate of a human or animal are well known. Examples of these types of devices are described in U.S. Pat. Nos. 4,248,244; 4,540,001; and 4,898,182. Generally, most heart rate monitors use electrodes to sense voltage fluctuations on a person's skin. The signal thus sensed is then amplified and passed through a suitable filter for filtering out the biopotential signals unrelated to heart rate. The frequency of the residual signal is then determined and displayed as heart rate (in beats per minute).
Autocorrelation has been used to isolate heart rate from noisy biopotential signals in certain ideal conditions, such as found in hospitals. See U.S. Pat. No. 4,403,184. However, noise, particularly periodic noise, reduces the reliability of autocorrelation. This problem is particularly acute during physical exercise. Especially troublesome are repetitive exercises, such as performed on an exercise bike, stair climbing machine or treadmill. In repetitive exercises, muscles tend to be exerted in a rhythmic or periodic fashion, thereby generating a periodic biopotential signal which cannot easily be distinguished from heart rate. Moreover, because heart rate tends to vary during exercise, the steady rhythm of repetitive exercise motion may generate a stronger autocorrelated signal than actual heart rate.
Also problem prone is the interface between the human subject and the electrodes which are used to detect biopotential signals. In practice, persons exercising prefer to not be encumbered by the type of bracelet, harness or clip which is typically used to make contact between a person and the electrodes. Ideally, the electrodes are placed on a handlebar or other location which the person touches as a matter of course while exercising. Under these conditions, however, the person is likely to remove his or her hands from time to time, thereby complicating efforts to accurately and continuously monitor heart rate.